This invention relates to a resin-coated carrier for use in two-component electrophotographic developers comprising a carrier component together with a toner component.
In one of the electrophotographic methods, a dry, two-component developer comprising a toner component in combination with a carrier component is used in the magnetic brush developing method to render visible an electrostatic latent image formed on a photoreceptor.
Generally, the developer comprises a mixture of relatively fine toner particles and relatively coarse carrier particles. The electrostatic attractive force between the opposite polarities generated by contact of these particles holds the fine toner particles on the surface of the coarse carrier particles. When the thusly charged developer is brought into contact with the electrostatic latent image formed on the photoreceptor, part of the charged toner particles are electrostatically attracted and deposited on the latent image to produce a corresponding visible image.
Therefore, the toner particles should have an appropriate triboelectric property so that they hold a sufficient charge to ensure the selective deposition of the toner particles of the latent image.
In the magnetic brush developing method, carrier particles carrying the toner particles are attracted to a magnet roll to form a magnetic brush thereon. The magnetic brush contacts the latent image formed on the photoreceptor as the magnet roll rotates. The carrier particles serve to carry the toner particles to transport them to a developing area of the photoreceptor. As such carrier particles, iron oxide, magnetite or ferrite particles have heretofore been used, and these particles are coated with a resin film.
The carrier particles mentioned above, however, have a high apparent density which increases the required torque for rotating the magnet roll of the developing device. In addition, the specific surface area of the carrier particles is small. Consequently, the retention ratio of toner on the carrier particles may not be increased, and a lower retention ratio of toner may result in unnecessary deposition of toner on the area where the latent image is not formed. Consequently, the quality of the final reproduced copies becomes poor due to the soiling of the background with a significant amount of toner. This is called fog.